Preparation of polyvinyl alcohol with water washing of gel



United States Patent 2,759,914 PREPARATION OF POLYVINYL ALCOHOL WITHWATER WASHING OF GEL William '0. Kenyon, George P. Waugh, and Erle W.Taylor, Rochester, N. Y, assignors to Eastman Kodak Company, Rochester,N. Y., a corporation of New Jersey No drawing. Original application July2, 1948, Serial No. 36,816, now Patent No. 2,642,420, dated June 16,1953. Divided and this application April 20, 1953, Serial No. 349,942

14 Claims. (Cl. 26.0-'91.3)

This invention relates to polyvinyl alcohol, and more particularly toimprovements in production and purification of polyvinyl alcohol. Morespecifically, the invention relates to an improved method of preparinghighly purified and substantially fully hydrolyzed polyvinyl alcoholwhich is suitable for use in the production of photographic emulsions orfor use in other instances which require such refined polyvinylalcohols.

This application is a division of our copending application S. N. 36,816filed July 2, 1948, and entitled Preparation of Polyvinyl Alcohol, nowPatent 2,642,420 of June 16, 1953.

Heretofore several methods have been described for making polyvinylalcohol and partial derivatives thereof. Nearly all of the desirablemethods basically include a procedure for the deacetylation of polyvinylacetate using acidic or alkaline reagents. The patents herein discussedare illustrative of such prior art methods for preparing polyvinylalcohol. In U. S. Patent 1,672,156 of June 5, 1928, polymerized vinylalcohol is prepared by treatment of polymerized vinyl acetate withalcoholic potash. The resulting polyvinyl alcohol exists as a voluminouspowder which is water soluble. Films or sheets can be prepared bydissolving this powder in water and evaporating this water therefrom. InU. S. Patent 2,109,883, of March 1, 1938, it is stated that the reactiondescribed in U. S. Patent 1,672,156 supra, is fundamentally asaponification reaction and the result is to have the desiredpolymerized vinyl alcohol powder contaminated to a more or less extentwith alkaline salt which must be removed if a pure polyvinyl alcohol isdesired. According to U. S. 2,109,883, this contamination is minimizedby using absolute alcohol and only catalytic amounts of alkali or acid.In U. S. Patent 2,227,997 of January 7, 1941, the reaction described inU. S. Patent 2,109,883, supra, is discussed, i. e., the catalytichydrolysis of polyvinyl esters comprising reacting the polyvinyl esterwith an alcohol in the presence of a small amount of an alkali or asmall amount of an acid as the hydrolysis catalyst. The chief productsof the reaction are polyvinyl alcohol and methyl acetate. Because thecatalyst is present only in a small amount, it is stated that nosignificant contamination of the product occurs and further purificationis not required. Notwithstanding the improvement by using less catalyst,it is clear that since no purification step is disclosed, the polyvinylalcohol still contains an appreciable amount of impurity resulting fromthe catalyst. In U. S. Patent 2,227,997, the patentee, an associate ofthe patentees of the abovementioned patents, describes an improvementover their former procedures. This comprises dissolving the polyvinylacetate in a small amount of the alcohol and adding a small amount ofalkali to form a plastic doughy mass which, on mechanical working, i.e., kneading, is converted to a slurry of the hydrolysis product, andthe slurry on further working is formed into a powder. It is emphasizedin that patent the solvent alcohol should be anhydrous if a fullyhydrolyzed polyvinyl alcohol is desired. If a partially hydrolyzedproduct is to be made, the reaction may be interrupted by neutralizingthe catalyst, or by 2,759,914 Patented Aug. 21, 1956 diluting thereaction by the addition of water. The prodduct is obtained in powderform by distilling olf the methyl acetate and other volatileconstituents. It is stated that the amount of catalyst present in thefinal product does not appreciably contaminate it. Nevertheless, theproduct does contain catalyst impurities and sodium acetate.

We have found that such prior methods of producing polyvinyl alcohol maynot give a sufiiciently pure product so that it may be employed as asubstitute for gelatin in the manufacture of photographic film andpaper. In making these or comparable photographic articles, extremelysmall quantities of contaminants have a very great influence upon thequality of emulsions in which the polyvinyl alcohol is employed. Thus,we have found that even the small contamination mentioned in the abovediscussed patents, while permissible for many uses, may interfere ifpresent in polyvinyl alcohol employed in photographic emulsions.

Also in electrical applications it is desirable to have a polyvinylalcohol in a highly purified state so that it is substantially free fromconductive salts.

An object of the present invention is, therefore, a method of preparinga highly pure polyvinyl alcohol in the form of a gel which is of suchphysical form, strength, and chemical composition that it is resistantto water, i. e., the gel will not dissolve in water at moderatetemperatures and can, consequently, be washed with Water to removeimpurities therefrom.

Another object of the invention is a polyvinyl alcohol gel which can bewashed in water at moderate temperatures without dissolving therein.

Another object is a method of preparing polyvinyl alcohol by which theacetyl content thereof may be controlled.

Still another object is a polyvinyl alcohol which has an extremely lowacetyl content.

Another object is a method of preparing polyvinyl alcohol by which theviscosity thereof may be controlled.

Another object is a polyvinyl alcohol which has a high viscosity.

Still another object is a method of preparing polyvinyl alcohol gels bywhich the solids content is controlled.

Yet another object is a method of preparing polyvinyl alcohol gels bywhich the solubility temperature is con trolled.

Other objects of the invention are methods for preparing polyvinylalcohols which have one or more of the above-indicated properties invarious combinations as may be desired. Other objects will appearhereinafter.

In accordance with the broad concept of the invention a polyvinylalcohol in a gel form which is resistant to dissolving or sliming bywater may be prepared by adding a deacetylation catalyst, such as anacid or alkali catalyst, to a solution of polyvinyl acetate, preferablywhile the solution is vigorously stirred. An alcohol solution of thepolyvinyl acetate is preferred, and methyl alcohol is the preferredalcohol. The alcohol may contain up to 10% water but an increase inwater content in the alcohol tends to the production of a less firm gel.The resulting deacetylation reaction produces polyvinyl alcohol in gelform which is resistant to water at moderate temperatures. Its washingcharacteristics and other characteristics can be modified by subsequenttreatments such as described hereinafter. This gel is then washed inwater to remove solvent, by-products of the reaction, and any unreactedcatalyst, or catalyst residues. The washing may be inter mittent, orcontinuous, or countercurrent.

In accordance with another feature of the invention, the resistance ofthe polyvinyl alcohol to water may be further improved by removal ofresidual acetyl groups from the polyvinyl alcohol chain of thedeacetylation product.

This may be accomplished by treating the polyvinyl alcohol gel, formedas above described, with additional deacetylation catalyst. Alcoholicalkali, aqueous alkali, aqueous acid, or alcoholic acid may be employedfor this additional deacetylation. In general, we have found thatemploying relatively large amounts of alkali deacetylation catalyst inthe initial and subsequent deacetylation operations results in apolyvinyl alcohol of low acetyl content. We have also foundthat bypermitting the gel to synerese and separating the syneresed liquidby-prodnot before employing additional deacetylation operations willpromote the production of low acetyl content polyvinyl alcohol. We havealso found that the absence of water in the original deacetylationmixture tends to permit the production of low acetyl content polyvinylalcohol.

If a polyvinyl alcohol is desired, which, while still washable withwater, contains more acetyl groups on the polyvinyl alcohol chain,operations contrary to those just described Will result in a higheracetyl content. Thus the presence of water in the initial deacetylationmixture, elimination of secondary deacetylation treatment, employingsmaller amounts of alkaline catalysts, employing a relatively shortinitial deacetylation period, and employing a weak alkali in the seconddeacetylation treatment, will permit more lacetyl to remain on thechain.

In accordance with another feature of the invention, we have found thatthe water absorption of the polyvinyl alcohol gel may be reduced byreducing the acetal content of the polyvinyl alcohol. There is usuallylittle or no acetal in polyvinyl alcohol made by alkaline deacetylation(less than 0.2% but when hydrochloric acid is employed as the catalyst,as much as l to 2% by weight acetal, calculated as acetaldehydemay bepresent. This may come from hydrolysis of a little residual monomer,resulting in condensation of the acetaldehyde thereby formed with thepolyvinyl alcohol under the influence f the acid conditions. This smallamount of acetal is sufficient to have some effect on the waterabsorption of the gel while washing, and also on the softening by waterof photographic emulsions made using this polyvinyl alcohol. We havefound that the acetal content of polyvinyl alcohol made in this way canbe greatly reduced by a preliminary steaming of the polyvinyl acetate inthe form of beads. We have also found that polyvinyl alcohol of the acidtype having a still lower acetal content can be made by conducting theoriginal deacetylation with alkali, and subsequently treating withalcoholic or aqueous acid.

In accordance with still another feature of the invention we have foundthat a polyvinyl alcohol of a relatively high viscosity may be preparedby conducting the original deacetylation with hydrochloric acid, or bytreatment of a gel originally made by alkali deacetylation With aqueousor alcoholic hydrochloric acid.

We have also found that the clarity .of the gel may be improved by atreatment with dilute acid which is not strong enough to cause a changein intrinsic viscosity. The color of the gel may be improved bybleaching with bleaching agents such as sulfur dioxide, hydrogenperoxide, chlorine, sodium hypochlorite and sodium bisulfite. The colormay also be improved by employing additional alcoholic deacetylationsteps after the initial deacetylation.

It is desirable to produce a gel having a high solids content ofpolyvinyl alcohol and this may be accomplished, we 'have found, byreducing the acetyl or acetal content of the gel, employing a prolongedsyneresis time, and conducting the secondary deacetylation operations inthe presence of water, methanol and methyl acetate.

It is also desirable, for photographic use, to produce a polyvinylalcohol having a minimum of reducing action, as shown by the ammoniacalsilver nitrate test.

In connection with photographic emulsion work, it is sometimes desinablethat the polyvinyl alcohol have a high minimum solubility temperature.This property is attained by reduction of the acetyl or acetal of thegel, and

may be enhanced by prolonged aging of the gel, or even aging at elevatedtemperatures.

A water washable polyvinyl alcohol gel may be prepared according to ourinvention, by either an alkali or acid catalyst. Alkali catalysts suchas sodium hydroxide or potassium hydroxideare preferred although otheralkali catalysts such as sodium methoxide or lithium hydroxide may beemployed. Hydrochloric acid is the preferred acid catalyst but goodwashable gels are produced using sulfuric and other strong acids.

Methyl alcohol is the desirable alcohol to employ in making up thesolution of polyvinyl acetate since the methyl acetate formed is quitesoluble in water and, therefore, is easily washed from the gel, butethyl alcohol or other higher alcohols may be employed.

The washing of the polyvinyl alcohol gel may be made with tap water,distilled water, or demineralized water, or the gel can be given apreliminary wash in water, then a dilute acid wash such as with diluteacetic or hydrochloric acid and then further washed with water. Thisweak acid treatment results in a gel which melts to a very clearsolution and which is especially adaptable for use in preparingphotographic emulsions.

After the initial gel formation, the gel may be subdivided, if desired,to facilitate further treatment in accordance with the invention. Suchsubdivision may be achieved, for example, by means of a grinder, a diceror a mill.

When washing the gelsit is desirable to employ water at a temperature offrom 4 to C. since the gel may be somewhat soluble in water above thisupper temperature.

The various features of our invention are further described in thefollowing examples:

Example 1.Acid deacetylation at 25 C.-Efiect 0f syneresis timeomconcentration of gel.

To 48.25 pounds of a methanol dope containing 22 percent by weight ofV-6O polyvinyl acetate were added, with thorough stirring, 2400 cc. of 6N aqueous hydrochloric acid. Six samples, see Table I, of the abovemixture, weighing about 6.4 pounds each, were placed in separate closedcontainers and set in a bath at 25 C. After about 72 hours, all thesamples had begun to form white, opaque gels. The samples were left inthe bath foradditional syneresing times as shown in Table I. The weightsof the gel and syneresed liquids were determined. The gels were slicedand washed in distilled water at room temperature until no further testfor chloride ion was obtained. A portion of each washed gel was blottedfree from surface water and the solids content, i. e. polyvinyl alcoholdetermined by drying at 110 C. Minimum solubility temperatures were alsorun on the washed gels. Minimum solubility temperature (M. S. T.) isdefined as the temperature at which a sample of a gel when vigorouslystirred becomes a smooth solution. In the examples the terms V.6, V-60,etc., are employed in the usual sense, that is, the number refers to theviscosity in centipoises at 25 C. of a benzene dope containing 8.60grams of the polyvinyl acetate per cc. of the dope. These data are shownin Table I.

It will be observed from the above table that prolonged syneresis timecaused the gel to shrink, giving more syneresed liquid, and giving a gelwhich, on being washed in water, did not dissolve or swell unduly.

Example 2.-Alkaline deacetylation Into 2000 grams of a 22 percent (byweight) methanol dope of V-60 polyvinyl acetate was stirred a solutionof 8 grams of sodium hydroxide in 620 grams of methanol. The mixturebecame dark red and, after minutes, began to gel rapidly. After asyneresis period of 24 hours at room temperature, the gel weighed 1730grams, and 850 cc. of syneresed liquid had separated. The gel was slicedand washed for 90 hours in a number of changes of distilled water. Thewashed gel catalyst (a 0.5 normal solution of sodium hydroxide inmethanol) in the proportion of 1 cc. of catalyst solution per 10 g. ofdope. Color developed, and the dopes set to firm gels. They were allowedto synerese for 40 hours, then were sliced and Washed at roomtemperature in distilled water for 3 days. These gels were examined asto solids content and M. S. T. These data are summarized in Table III.It is apparent that by employing polyvinyl acetate of higher viscositiesthat a shorter gelation time and a higher solids content results.

Example 3.Alkaline deacetylation with less catalyst This is a parallelexperiment to Example 2, but using 1968 grams of the methanol dope ofpolyvinyl acetate and a weaker alkaline catalyst solution comprising0.98 g. of sodium hydroxide in 632 g. of methanol. Gelation required 2.4hours. After 24 hours syneresis, the gel weighed 2265 g. and thesyneresed liquid was 220 cc. After slicing and washing in distilledwater for 90 hours, the gel weighed 3457 g. It was a clear, highlyswollen, slimy product containing 2.6 percent solid.

Comparison of Examples 2 and 3 shows the advantage of using a relativelylarge amount of alkaline catalyst as in Example 2.

Example 4.-Syneresis time seriesAcid deacetylation at 40 C.

To 18,000 g. of 22 percent methanol dope of V-60 polyvinyl acetate wereadded, with thorough stirring, 1968 cc. of a mixture of equal parts ofconcentrated hydrochloric acid and water. The dope was split into sixportions, each in a covered bottle, and thesewere placed in a 40 C.bath. In about 18 hours these dopes turned to gels.

One of these samples was removed after an additional 24-hour syneresisperiod. The tough, opaque white cake of polyvinyl alcohol weighed 2695g. The syneresed liquid weighed 496 g. The gel was sliced and washed ina number of changes of distilled water as before. The other bottles wereopened after longer syneresis times, and the gels similarly sliced andwashed with water. The solids contents did not vary much and the minimumsolution temperatures increased only slightly, indicating that 1 to 2days syneresis time with this Example 5.Efiect of viscosity (molecularweight) of polyvinyl acetate on physical properties of washed polyvinylalcohol gel-Alkaline deacetylation Six 22 per cent dopes of polyvinylacetate, of various viscosities, indicated in the table as V2.5, V-6,etc.,

were made up in methanol. To each was added alkaline Example 6.Effect ofincreased syneresis time and temperature, alkaline deacetylation To anumber of small samples of 24 per cent dope of V- polyvinyl acetate inmethanol was added alkaline catalyst, in the proportion of 25 g. of a3.71 per cent methanol solution of sodium hydroxide per pound of dope.These set to gels within a few minutes. They were set away at differenttemperatures for varying syneresis times, after which the gels weresliced and washed in running tap water. The gels were then ground in ameat grinder and given an additional washing period of 16 hours inrunning tap water, then were drained for one day to remove most of thesurface water. They were analyzed for solids content and for per centpoly-.

vinyl acetate. The results of the runs at 8, 30 and 50 C. are shown inTable IV.

TABLE IV at 8 C. at 30 C. at 50 C.

syneresis, Hours Percent Percent Percent Percent Percent Percent SolidsP Ac Solids PVAc Solids PVAc 1 These two samples partly dried out duringthe syneresis.

As shown in Table IV, both increased time and increased temperature arehelpful in obtaining high solids and low polyvinyl acetate content. Thefact that two of the samples at 50 C. dried out during the syneresis andgave unusually high solids, is in line with the results described inExamples 8, 9, 10, 11 and 12, where liquid was squeezed out of the gels.

The above samples did not vary significantly in viscosity. Specificviscosities in water taken at a concentration of 0.100 gram per 100 cc.of solution ranged from 0.101 to 0.112; corresponding to inherentviscosities taken in Water at a concentration of 0.250 gram per 100 cc.of solution, of 0.96 to 1.06. These two viscosity functions may bedesignated as (1; sp) and {1 respectively. Inherent viscosity is thesame mathematical function as intrinsic viscosity, but measured at afinite concentration.

Example 7.Eflect of increased catalyst, at various times andtemperatures, alkaline deacetylation A series of experiments was runsimilar to Example 5 above, except that approximately twice as muchcatalyst was used, that is, 50 g. of 3.71 percent solution of sodiumhydroxide were added per pound of dope. This series was run at 30 and at50 C. The solids and polyvinyl acetate determinations were made afterwashing, etc., as in Exam- It will be observed that increased time andtemperature have an effect here similar to that in Example 6. Comparisonof Examples 6 and 7 show that the use of more catalyst is definitelyhelpful in obtaining increased solid and low polyvinyl acetate content.

The above samples did not vary significantly in viscosity, either amongthemselves or from the products of Example 6 in Table IV above. Observedvalues were (1; sp) of 0.107 to 0.111, or {1 (or inherent viscosity) of1.02 to 1.05.

Example 8.Deacetylation using a mixer In a stainless steel mixing millequipped with Sigma blades, of approximately 4 gallons working capacity,were placed 8 lbs. 2 oz. of V22 polyvinyl acetate in the form of smallspheres or beads, and 18.9 lbs. of methanol. After this was all doped,1350 g. of 3.71 percent solution of sodium hydroxide in methanol wereadded, with continued mixing. After 11 minutes a gel formed. The mixerwas run for an additional two hours, after which the product waswithdrawn. It was in'the form of damp flakes, with considerablesyneresed liquid. A sample of these flakes was allowed to stand in thesyneresed liquid for one day at room temperature, followed by three dayswashing and one days draining. The solids on this sample was 21.39percent; the polyvinyl acetate content 1.37 percent. The M. S. T.observed was 80 C. which is higher than that obtained by the methodsshown in the other examples and appears due to the milling action.

Example 9.Deacetylatin using a mixerUse of fresh alkaline liquid Part ofthe polyvinyl alcohol flakes obtained from the mixer in the aboveexperiment were drained free of excess syneresed liquid, and werecovered with a methanol solution made up to represent about the originalalkalinity, that is, containing 50 g. of 3.71 percent methanol solutionof sodium hydroxide, per pound of methanol. After one day at roomtemperature, followed by three days of washing and one day of draining,this sample showed a solids content of 26.61 percent and a polyvinylacetate content of only 0.74 percent. The M. S. T. was 88 C.

This indicates the advantage of discarding the syneresed liquid andusing a fresh solution of catalyst in order to obtain a product havinghigher solids and lower acetyl.

Example 10.-Deacetylation using a mixerUse of fresh alkaline liquidcontaining stronger alkali In the same mixer as was used in the aboveexample, polyvinyl alcohol shreds were made by a similar alkalinedeacetylation. The mixer was stopped 1.5 hours after addition of thecatalyst, and the material withdrawn. About lbs. of damp flakes ofpolyvinyl alcohol were thus obtained, and over 16 lbs. of syneresedliquid. The damp flakes were covered in a crock with an alkaline liquidcorresponding to about 3 times the normal original alkalinity. Thiscomprised 155 g. of sodium hydroxide in 33 lbs. of methanol. Afterstanding thus for one day, with occasional stirring, the flakes werewashed for four days in cold running water and drained for one day.Analysis showed 26.32 percent solids and 0.43 percent polyvinyl 8acetate. The M. S. T. was 88 C. and the {n} was 0.72.

This shows that by using methanol containing more alkali to replace thesyneresed liquid the acetyl can be further decreased although comparisonwith Example 8 shows that in these cases no significant change in solidsor M. S. T. was observed.

Example 11.Deacetylation using a mixerShort reaction time In the samemixer the original deacetylation with formation of polyvinyl alcoholflakes was done as in Example 10 above. The flakes were put to washingin cold water 2.3 hours after the addition of sodium hydroxide. Afterfour days washing and one days draining, the solids was 19.63 percent,the polyvinyl acetate content 1.66 percent. The M. S. T. was 80 and the{1;} was 0.76.

Example 12.Deacetylation using a mixerReplacement of alkaline liquidwith acid A dope of polyvinyl acetate was converted to polyvinyl alcoholflakes using alkaline catalyst as in Example 11 above. About 2.5 hoursafter the addition of the alkaline catalyst, the polyvinyl alcoholflakes were drained and covered in a crock with 25 lbs. of a solution of3 lbs. of concentrated hydrochloric acid in 30 lbs. of methanol. Afterstanding for one day, with occasional stirring, the flakes were washedin cold running water for 3 days and drained for one day. Analysisshowed 25.65 percent of solids and 1.10 percent polyvinyl acetate. TheM. S. T. was 86 and the {17} was 0.85. By comparison with Examples l0,ll, 13, l4, l5 and 16, it will be seen that this viscosity is typicalalcohol made by acid deacetylation, rather than alkaline, of V-22polyvinyl acetate. Thus, at least one clearly recognizable property ofacid-type polyvinyl alcohol has been imparted by soaking thealkaliformed flakes in acid.

Example 13.Alkaline deacetylation-Use of hydrogen peroxide to avoidcolor To 1900 g. of a 30 per cent dope of V-22 polyvinyl acetate inmethanol, were added 6.3 cc. of 30 per cent hydrogen peroxide, withcontinuous stirring. After 10 minutes were stirred in 315 g. of alkalinecatalyst, a 3.71 per cent solution of sodium hydroxide in methanol. Thedope turned red immediately upon addition of the catalyst, but thiscolor rapidly faded and it became colorless within a few minutes. Eightminutes after adding the catalyst, a colorless translucent gel hadformed. After 5 days at room temperature, this gel was sliced, washed,ground, rewashed, and drained for one day. Analysis showed 13.13 percent solids and 1.77 per cent polyvinyl acetate. It showed an {n} of0.75 in water.

The light color of this polyvinyl alcohol was due to the use of hydrogenperoxide, as the particular batch of polyvinyl acetate used in thisexperiment had, on previous occasions, given a much darker red uponaddition of the alkaline catalyst, and the resulting polyvinyl alcoholgel was a fairly deep yellow.

It will be noted that more alkaline catalyst is necessary when hydrogenperoxide is used in this deacetylation. Use of the normal amount ofsodium hydroxide, the proportion used in Example 8, gives a softer gelof higher acetyl content in the presence of a similar amount of hydrogenperoxide.

Example 14. Acid deacetylationUse of hydrogen peroxide for bleaching To1900 cc. of 30 per cent methanol dope of V-22 polyvinyl acetate, of atype which gave much color with alkaline catalyst, were added withstirring, 6.3 cc. of 30 per cent hydrogen peroxide. After a few minutes,208 cc. of a 1:1 mixture of concentrated hydrochloric acid in waterstirred in and the dope placed in a bath. After about 54 hours, theresulting gel was washed in cold running water for 5 days, ground in ameat grinder, and

re-washed for 4 hours more. Analysis showed 11.72 per cent solids, 1.16per cent polyvinyl acetate, 1.29 per cent acetal calculated asacetaldehyde, and an {n} of 0.83.

Example 15.Plyvinyl alcohol of low acetal content by acid hydrolysisUseof steamed polyvinyl acetate V-22 polyvinyl acetate was made bypolymerization in the form of small spheres or beads of approximately to40-mesh size. This was done by polymerization with agitation in thepresence of water, using starch as the dispersing agent. When thepolymerization was complete, an excess of steam was passed into thereaction mixture, with continuous stirring, thus keeping the temperatureat approximately 100 C. for 10 minutes. The polyvinyl acetate beads werethen washed in cold running water and dried at room temperature.

A 30 per cent dope was made of this polyvinyl acetate in methanol. Tothis was added a mixture of concentrated hydrochloric acid and water 1:1in the proportion of 49.6 cc. per pound of dope. This was maintained at50 C. for 54 hours, during the first 18 hours of which time it set to afirm white gel. The gel was sliced, washed, in running water for 5 days,ground in a meat grinder, rewashed for 4 hours, and drained for one day.This gel had 12.68 per cent solids. Analysis showed 1.21 per centpolyvinyl acetate and 0.46 per cent acetal calculated as acetaldehyde,on a dry basis. The {n} of this polyvinyl alcohol in water was found tobe 0.81.

Example 16.P0lyvinyl alcohol by acid hydrolysis In an experiment exactlylike Example 16 above, except that the polyvinyl acetate beads werewashed directly instead of being steamed, the gel was of similarappearance and contained 11.70 per cent solids. Analysis showed 1.27 percent polyvinyl acetate and 1.83 per cent acetal calculated asacetaldehyde, on a dry basis. The {n} of this polyvinyl alcohol wasfound to be 0.82 in water.

Several polyvinyl acetates, some of which had been steamed in head formand some which were not, were deacetylated by acid or alkali catalyst,the syneresed gels washed, drained and analyzed. The results are shownin Table VI.

TABLE VI Polyvinyl Acetate Polyvinyl Alcohol Gel C 1 Per; Peri-z Acetall T Steamed ata st cen cen as per- 1;

we y Solids IVAc c t c Example 17.-Acid deacetylation using sulfuricacid To 100 g. of a 30 per cent methanol dope of V-22 polyvinyl acetatewas added a mixture of 5.5 g. of sulfuric acid and 13.8 g. of methanol.The dope was maintained at 50 for 48 hours, during the first 6 hours ofwhich time it set to a gel and began to synerese. The gel was sliced,washed, ground, re-washed, and drained for one 1 day. The gel had 6.66per cent solids, and a M. S. T. of 58. Analysis showed 1.04 per centpolyvinyl acetate on a dry basis.

Example 18.Deacetylation of a copolymer of vinyl acetate and vinylchl0rideAlkaline catalyst To 137 g. of a 22 per cent methanol dope of acopolymer of vinyl acetate and vinyl chloride, containing 2.4 per centof the latter, were added 15.1 g. of a 3.71 per cent methanol solutionof sodium hydroxide. A light pink color developed, and a gel formed in10 minutes. After 48 hours at room temperature, the gel was washed,etc., as in Example 17. The gel looked much like ordinary polyvinylalcohol gel. It melted at 72 to a cloudy dope, which was made clear bythe addition of half its volume of methanol. Analysis showed 12.33 percent solid and 1.90 per cent polyvinyl acetate. The gel blackened whendried in a vacuum oven at 60. The dry product contained 2.5 per centchlorine. The calculated value, assuming no hydrolysis of vinylchloride, is 2.6 per cent chlorine.

Example 19.--Deacetylati0n with sodium methoxide An experiment was madeby the general method of Example 7 in which the sodium hydroxidedeacetylation catalyst was replaced by an equivalent amount of sodiummethoxide, made by reacting sodium and methanol. A slightly lowerpolyvinyl acetate content was obtained, 1.3 per cent compared with 1.5to 1.6 for the checks using sodium hydroxide.

Example 20.-Use of additional steeping liquids A dope of V- polyvinylacetate was converted to polyvinyl alcohol as in Example 12 in which agel was set up in a Sigma blade mixer using alkaline catalyst, which wasthen drained and a part of the gel covered with a mixture of three partsconcentrated hydrochloric acid and 30 parts of methanol by weight. Itwas then washed in running water. Another portion of the same gel, afterbeing removed from the mixer, was steeped in a mixture of 141 grams ofsodium hydroxide and 33 pounds of methanol for one day, then steeped inmethanolic hydrochloric acid as above, then washed in running water.

The following analyses show that the acetyl, calculated as polyvinylacetate and the acetal, calculated as acetaldehyde, were considerablyreduced by this intermediate treatment.

Example 21 .T my method with alkaline catalyst To 45 pounds of a 24%dope of V-22 polyvinyl acetate in methanol was added with thoroughmixing 5 pounds of alkaline catalyst, comprising a 3.71% solution ofsodium hydroxide in methanol. After 14 minutes the dope began to set toa gel. After 2 minutes more it was transferred to a tray. After 3minutes in the tray it was sliced and ground in a meat grinder, whichrequired ten minutes. After standing for an hour at room temperature,considerable liquid had syneresed out, which was discarded. The gel wascovered with a solution of 141 g. of sodium hydroxide in 33 pounds ofmethanol. After two days at room temperature, with occasional stirring,the gel was washed with cold running water for two days and drained.

I Example 22.Tray method with acid treatment Two pounds of the aqueousgel prepared as described in Example 21 was soaked for two days withoccasional stirring in a mixture of 200 cc. of concentrated hydrochloricacid and 1800 cc. of water. It was then washed for two days and drained.

Example 23.-Tray method with alkaline catalyst To 40 pounds of a 30%dope of V-9 polyvinyl acetate in methanol was added with thoroughstirring 8 pounds and 14 ounces of a 3.71% solution of sodium hydroxidein methanol. Seven minutes later the dope began to gel. After oneadditional minute it was transferred to a tray. After an additional fourminutes it was sliced and ground as above, which required ten minutes.After standing an hour at room temperature, the gel had syneresedconsiderable liquid, which was drained off and discarded.- The gel wascovered with a solution of 377 grams of sodium hydroxide in 47 pounds ofmethanol, and allowed to stand at room temperature with occasionalstirring for two days. It was then washed for two days in running waterand drained.

Example 2'4.Tray method with acid treatment Two pounds of the aqueousgel of Example 23 was soaked in a mixture of 200 cc. of concentratedhydrochloric acid and 1800 cc. of water for two days. It was then washedand drained.

The analysis of the products of Examples 21 to 24 is shown in thefollowing table.

TA B LE VII Example Type Percent Percent {1 M. S. T.,

Solids P. V. Ac degree Alkali V22 16.02 0. 66 0. 69 70 Acid V-22.. 17.61 0,14 0. 78' 76 Alkali V-9. 13. 99 0. 58 0. 54 68 Acid V-9 16. 33 0.270.61 71 Example 25 .Steeping gels in alkali In the tray method, two daysof steeping in methanolic alkali are desirable to obtain low acetylcontent, as shown in the following table showing analytical results ongels made this way, which were steeped for three dilferent lengths oftime in alkaline methanol.

TA 13 LE VIII Example 26.Use of S02 as a bleach Samples of aqueouspolyvinyl alcohol gel of approximately 15% solids content were made byalkaline deacetylation of V9 and V22 polyvinyl acetate respectively inmethanol, during 5 days at room temperature, followed by Washing inwater. This follows the general method of Example 7.

Samples of each of these gels were soaked for one day in nearlysaturated solutions of sulfur dioxide in water, then washed in water.During the treatment, it was observed that most of the bleaching tookplace during the first one to four hours. Comparisons were made of thewashed gels, as to color, minimum solution temperature, clarity of dopeformed by melting, and inherent viscosity in water.

The differences found in inherent viscosity between treated anduntreated gels are within experimental error. Dopes made from thetreated and untreated gels were also tested for reducing materials bythe ammoniacal silver nitrate test. In each case the sulfurdioxide-treated sample was found to be more free from reducing materials.

Thus the sulfur dioxide treatment is desirable from the standpoint ofappearance, clarity of dope, and absence of reducing materials. Theimproved clarity is no doubt due to removal of insoluble calcium saltssuch as carbonates. There are not adverse effects on the gel from thistreatment.

It will be observed in some of the examples that variations in thedeacetylation conditions do not affect the inherent viscosity of thepolyvinyl alcohol formed, within experimental error, except that thereis a noticeable and uniform difference between inherent viscosities ofpolyvinyl alcohol made by acid and by alkaline deacetylation. Themolecular weight of the polyvinyl acetate used is, of course reflectedin the inherent viscosity of the resulting polyvinyl alcohol. It is ofinterest that the products made by the use of the mixer although showinga much higher minimum solution temperature than products made by massivedeacetylation using comparable formulas showed little or no differencein inherent viscosity. After being melted or doped in hot waterpolyvinyl alcohols made by these two methods behaved similarly.

- The molecular weight of the original polyvinyl acetate also influencesthe properties of the resulting polyvinyl alcohol. We have found, otherconditions being equal, a higher molecular weight polyvinyl acetate willgive a polyvinyl alcohol of lower acetyl, and the washed gel willcontain less water and usually have a higher minimum solutiontemperature. These last two factors are not always met in practice,however, since as a matter of convenience a more dilute dope of thehigher molecular weight polyvinyl acetate may be used.

The concentration of the dope in which the gel sets up, that is,including both the volume of polyvinyl acetate and the volume of thecatalyst, affects the minimum solution temperature and the solidscontent of the gel in that the more concentrated the dope the higher thesolids content and the higher the minimum solution tempera ture, withinlimits.

The presence of water, even in relatively small amounts, in an alkalinedeacetylation tends to make the deacetylation slower and less complete,and results in a softer gel of lower solids content and of lower minimumsolution temperature. Water is present in the acid hydrolysis, but inthis case it does not have nearly as great an efiect.

The concentration of the catalyst, expressed as amount of sodiumhydroxide per unit weight of dope, influences the alkaline deacetylationquite markedly, higher catalyst concentration causing a more rapidgelling, and also lower acetyl, a higher solids content of the gel, anda higher minimum solution temperature. This is shown by a comparison ofExamples 2 and 3 and also 6 and 7 above. The concentration of acid has asimilar effect but is considerably less marked.

Higher temperatures favor more rapid reaction, higher solids content inthe Washed polyvinyl alcohol gel, and lower acetyl. This eifect is notextreme, however, and for the sake of convenience, room temperature maybe used especially when using alkaline catalyst. Comparison of Examples1 and 4, in which similar amounts of acid catalyst were used shows thebenefit of using elevated temperature in acid hydrolysis. Thetemperature attained in ordinary equipment is limited by the presence ofmethanol and methyl acetate which form a low-boiling constant boilingmixture.

The time of syneresis is definitely a factor, as shown in Examples 1, 6,7, 8, and 10. Longer time promotes higher solids content, higher minimumsolution temperature, and lower acetyl, but under any particular set of13 reaction conditions these qualities approach a limit, so thatextremely long syneresis times are not economical.

There appears to be a direct relationship between the solids content ofthe gel before washing and the solids content after washing. This isindicated in Examples 8, 9, 10, 11 and 12 in which considerably moreliquid than usual had separated out in early stages of thedeacetylation. These gels also when washed had quite a high solidscontent.

The temperature of the wash water affects the solids content of thewashed gel, but not very greatly within ordinary washing temperatureranges. For example, a polyvinyl alcohol made by alkaline deacetylationof V-22 polyvinyl acetate, washed in cold water, about C. had a solidscontent of 13.4% whereas a similar polyvinyl alcohol washed atapproximately 25 C. had a solids content of 16%. The efiect of washingtemperature on polyvinyl alcohol made by acid hydrolysis appears to beeven less. Of course, the wash water must not be too warm as thepolyvinyl alcohol will take up a great deal of water at temperaturesapproaching its minimum solution temperature.

The solids reported in the examples have been found to be quitereproducible, for instance, among gels made the same way and washed atthe same temperature, the solids of all usually agree within 10.5%. Wehave observed, however, that there is a considerable tendency of thewashed gel to lose water on standing, especially if it was washed incold water and then stands at room temperature. As an illustration, apolyvinyl alcohol gel made by alkaline deacetylation of V-80 polyvinylacetate contained 13.65% solids after one days draining, but afterstanding for five weeks at room temperature (approximately 25 C.) in aclosed bottle, the upper part showed a solids content of 18.01%. Theseparated water was present in the bottom of the bottle. This tendencyis reduced in the case of gels having relatively high solids content;the gels made by using a mixer showed little such separation of water.

The choice of the catalyst is a major factor in the preparation of thepolyvinyl alcohol. When the deactylations are done as in Examples 2, 3,5, 6, 7, 13 and 18 above, that is, not using extreme conditions, thealkaline procedure may give a product of slightly higher acetyl, a lowerintrinsic viscosity, and little or no acetal, whereas the acid processmay give slightly lower acetyl, a higher intrinsic viscosity and anappreciable amount of acetal. When used in the photographic emulsions,they also have different properties.

Although most of the examples show the deacetylation of polyvinylacetate by hydrochloric acid or sodium hydroxide, which is the operationwe have found most useful in our work, the processes should be capableof much wider application. Example 17 shows the use of sulfuric acid asa catalyst. We believe that any suitable strong acid could be usedsuccessfully, although some of the common acids, such as nitric orperchloric would, of course, be unsuitable for use in the presence ofmethanol.

The examples show the use of methanol but other solvents can be employedas ethyl alcohol or the higher alcohols, such as n-propyl, iso-propyl,n-butyl, sec-butyl and tert.-butyl. Experiments with these otheralcohols gave inferior results especially in that the gels were softer,of lower solids content, and showed a yellow or brown color. Also,methanol has the advantage that methyl acetate is quite soluble in waterand, therefore, is easily removed. Also, methanol is relatively cheapand available.

The process as indicated in Example 8 is applicable to copolymers ofvinyl acetate having comparatively small proportions of other materialsso that the product formed might be considered a modified polyvinylalcohol. Such materials are sometimes desired, in order to obtaindesirable solubility, insolubility or other properties. Example 18 showsthe deacetylation of a vinyl acetate-vinyl chloride copolymer to give amodified polyvinyl alcohol 14 which has the useful property oftolerating more organic solvent in its aqueous dope than does ordinarypolyvinyl alcohol. The process is also adaptable for other polyvinylesters, such as polyvinyl propionate, butyrate, etc.

It has been observed that any method or choice of conditions whichleaves a considerable amount of acetyl in the molecule gives a fairlysoft washed gel of relatively low solids content. The same tendency hasbeen observed regarding the presence of acetal, but to a less markedextent. This is probably because under ordinary conditions it is easy toleave considerable acetyl on the molecule but the amount of acetalformed is ordinarily rather limited. Experiments in which polyvinylalcohol has been deliberately modified by acetalizing some of thehydroxyls with acetaldehyde have shown this effect clearly, a fewpercent of acetal causing a great increase in the water absorption. Itis well known that partially deacetylated polyvinyl acetate containingfrom about 40 to polyvinyl aclohol, or polyvinyl acetals containing overabout 50% polyvinyl alcohol are water soluble.

We believe that improvements in the gel, that is, higher solids andgreater firmness come largely as a result of redicing these twoundesirable features, that is to say, as a result of obtaining purerpolyvinyl alcohol.

As shown above, sodium hydroxide in aqueous solution is quite eifectivein reducing the residual acetyl content of a polyvinyl alcohol gel.However, for certain purposes, such as more ready solubility, it may bedesired to remove only part of the acetyl and have a certain residualamount in the polyvinyl alcohol. This can be accomplished by varying theconcentration of the aqueous sodium hydroxide solution as shown inExample 27.

Example 27.C0ntr0l of residual acetyl by use of weak alkalies Apolyvinyl alcohol gel was made by alkaline deacetylation of a 30 percent methanolic dope of V-22 polyvinyl alcohol for 5 days at roomtemperature, followed by slicing, washing, subdividing further, andrewashing.

Samples of this gel were soaked with occasional stirring for one day invarious concentrations of aqueous alkali, followed by washing in water.Analysis showed the following:

It will be observed that the operating range for this purpose is fromabout 0.1 N to about 0.01 N. The former showed no significant differencefrom higher concentrations, and 0.01 N showed little change from theoriginal.

A similar effect can be obtained, with better control of residualacetyl, by using weaker alkalies such as ammonium hydroxide, trisodiumphosphate, sodium carbonate, a mixture of sodium carbonate and sodiumbicarbon' ate, etc. A pH of from about 10 to about 11 or 12 is mostuseful for this purpose. Generally speaking, the higher the pH the lowerthe residual acetyl. This is shown in Examples 28 and 29.

Example 28.-C0ntr0l of residual acetyl by use of weak alkalies A sampleof gel made as in Example 27 was similarly treated with 1 N ammoniumhydroxide and washed. Analysis showed 16.23 per cent solids and 0.45 percent polyvinyl acetate.

A polyvinyl alcohol gel was made by deacetylating with methanolic sodiumhydroxide a 30 per cent solution of V-22 polyvinyl acetate in 93.5 percent methanol (6.5 per cent water), and ground in a meat grinder Whilefresh. After standing for one day at room temperature, the syneresedliquid was discarded, and the gel was washed in water and drained.Samples of this gel were soaked for one day with occasional stirring, inan equal weight of dilute aqueous solutions of various alkalies as shownin the table below. The strengths listed are after allowing for dilutionby the gel. The gels were again washed in water, soaked in approximately0.1 N acetic acid, further washed, drained and analyzed. The results areshown below.

When alkaline gels are washed in tap water, especially if carbonate ionis in the gel, the washed gel gives a more or less hazy dope whenmelted. This is because of the formation of insoluble salts, principallycalcium carbonate. This can easily be avoided by treating the gel withacid, then washing. Dilute weak acid such as acetic can be used inalmost any concentration as from 0.001 N (0.006%) to 4 N (24%), althoughour preferred concentration is approximately 0.1 N (0.6%) acetic acid.Other acids, such as sulfurous or hydrochloric, in concentrations offrom 0.001 N to 0.1 N can be employed, the only requirement being thatthey must dissolve calcium carbonate, without harming the polyvinylalcohol gel. The use of acids to remove calcium carbonate, etc. from thepolyvinyl alcohol gel is also shown in Examples 26 and 29.

We have found that a gel may be set up in methanol, or methanolcontaining a little water, by addition of methanolic alkali to apolyvinyl acetate dope as usual, and the acetyl further reduced byaddition of aqueous alkali, without washing. The syneresis liquid may bediscarded or not. it, to avoid having to use enough alkali to saponifyall the methyl acetate formed during the original methanolysis. Thisremoval can be carried further by washing out part of it, as in Example30. Allowing the gel to synerese before this treatment is desirable,from the standpoint of firmness of the final washed gel. Our preferedsyneresis time is from one hour to one day, although it will be observedthat in Example 30 little or no syneresis time is allowed.

The alkali used may be a strong alkali as sodium hydroxide, or a weakalkali such as sodium carbonate, or a mixture. Sodium carbonate reactsto form sodium bicarbonate, so that when an excess is used, the pH isabout at the end of the reaction. Also, when most of the alkali issodium hydroxide and an excess of sodium carbonate used, the sodiumhydroxide is presumably used up, leaving excess sodium carbonate andformed sodium bicarbonate to determine the pH. This is useful to avoidunduly reducing the residual acetyl.

The benefit of the presence of organic liquid in the aqueous treatmentis that the solids content of the final washed gel is much higher thanwhen only water is used. Other liquids than the mixture of methanol andmethyl acetate could be used, but that would involve washing orotherwise removing the liquid present and Usually it is advantageous todiscard 5 16 replacing with other liquids. We have done this with freshmethanol, and obtained similar results, but it is obviously a greatsaving of chemicals and labor to use the liquid already present. Thesemethods are shown in Examples 30, 31, 32 and 33.

Example 30.-Alkali and acid treatment A 30% methanolic dope of V22polyvinyl acetate was deacetylated by addition of 4% its weight of a 10%solution of sodium hydroxide in methanol. The gel was ground while stillfresh, within 5 minutes of the time the gel set up, and covered with 2liters of an aqueous solution containing 106 grams of sodium hydroxide.After standing at room temperature for 2 days, with occasional stirring,the gel was washed, acidified with acetic acid, washed, and drained.Analysis showed 17% solids, 0.26% polyvinyl acetate, and a minimumsolution temperature of 83 C.

Example 31.Deacetylaii0n in presence of methanol methyl acetate usingsodium carbonate To a 25% dope of V-22 polyvinyl acetate, containingapproximately 4% water, was added 4% of its weight of a 10% solution ofsodium hydroxide in methanol. The resulting gel was sliced and groundwhile still fresh, and allowed to synerese for one day at roomtemperature. During this time the original gel syneresed to the extentthat it weighed 48% of its original weight. One pound of this gel wasadded to a solution of 21 2 grams of sodium carbonate in 2 liters ofwater, allowed to stand with occasional stirring for 2 days, washedbriefly in water, water and acetic acid were added sufiicient to makethe entire volume 0.1 N, and after one day washing was resumed. Theresulting polyvinyl alcohol gel was drained and analyzed. It was a firmgel of 22.12% solids, 1.27% polyvinyl acetate, with a minimum solutiontemperature of 72 C.

Example 32.Additi0nal deacetylation with sodium hydroxide and sodiumcarbonate An alkaline polyvinyl alcohol gel Weighing 40 pounds was setup as in Example 31, and allowed to synerese for 1.5 hours. During thistime it syneresed to of its original weight. The liquid was discarded,and the gel covered with a solution of 3 pounds 8 ounces of sodiumcarbonate and one pound 6 ounces of sodium hydroxide in 75 pounds ofwater. The mixture was allowed to stand at room temperature withoccasional stirring. Samples were removed at one and two days, washed,acidified with acetic acid, washed and drained. Analysis shows:

l-day sample, 24.03% solids, 0.82% polyvinyl acetate,

81 C. minimum solution temperature.

2-day sample, 25.20% solids, 0.62% polyvinyl acetate,

C. minimum solution temperature.

Example 33.Additi0nal deacezylation by sodium carbonate in presence of areduced amount of methanolmethyl acetate mixture Five pounds of freshlyground gel of V-22 polyvinyl alcohol, originally formed as in Examples31 and 32 was allowed to stand at room temperature for one hour. Withoutremoving the syneresed liquid, 2 pounds 14 ounces of water was added tocover the gel. The mixture was stirred occasionally and allowed to standat room temperature for one day. It was drained, leaving 3 pounds 5ounces of gel. This was covered with 5 pounds 2 ounces of water, allowedto stand for 3 hours with occasional stirring, and again drained,leaving 3 pounds 5 ounces of gel. This was covered with 5 pounds 2ounces of water, allowed to stand for 3 hours with occasional stirring,and again drained, leaving 4 pounds of gel. This was covered with 5pounds 2 ounces of water containing 118 grams of sodium carbonate,stirred, and allowed to stand at room temperature for one day. The gelwas drained, given two 3-hour Washes,

17 soaked for 18 hours in water containing acetic acid sufficient tomake the entire volume 0.1 normal, then washed to neutrality in tapwater and drained. Analysis showed 16.24% solids, 1.60% polyvinylacetate, and a minimum solution temperature of 70 C.

Example 34.-Soaking gel to increase solids A polyvinyl alcohol gel wasmade by adding A its weight of catalyst sodium hydroxide in methanol) toa 30% dope of V-22 polyvinyl acetate in 93.5% methanol (6.5% water). Thegel was ground while still fresh and allowed to synerese for one day atroom temperature. The syneresed liquid was discarded and the gel wasdivided in two parts. One part was covered with approximately 160% itsweight of water and allowed to soak with occasional stirring for 2 days,then washed in water and drained. The other part was washed without apreliminary 2 days soaking. Analysis showed the advantage of the soakingin giving increased solids content of the final gel.

Soaked-16.13% solids: 2.28% polyvinyl acetate 65 C. M. S. T.

Unsoaked.9.94% solids: 2.41% polyvinyl acetate 61 C. M. S. T.

Example 35.-' quivalency of alkali hydroxide TABLE XII HydroxideMoles/mole Percent Percent PVAc Solids PVAc Example 36.Specificconductivity of gels Samples of polyvinyl alcohols made in accordancewith our invention were ashed, using sulfuric acid to convert anymetallic salts to sulfate. The percent ash as sulfate is shown in thetable below. Samples of the same polyvinyl alcohols were dissolved byheating and stirring in boiled distilled water to make 2.0% dopes. Thedopes were filtered, and their resistance was measured using aWheatstone bridge. The specific conductivity of the various dopes andthe water which was used in making them is shown in the table below: Twounwashed samples and two commercial samples were similarly ashed anddoped for conductivity measurements for comparison. The table clearlyindicates the improvement obtainable by washing in accordance with ourinvention:

Our improved process thus permits the production of highly purifiedpolyvinyl alcohol without employing the steps of precipitating, drying,and removal of solvent by distillation, which steps heretofore employedin this field increase the cost or" operation through handling and use01' expensive equipment. Handling the polyvinyl alcohol in gel form andeliminating the precipitating, drying and distillation steps alsoeliminates the possibility of potential contamination during suchprecipitation, drying and distillation steps. Especially important inthe production of polyvinyl alcohol for use in photographic emulsions isthe elimination of the drying step formerly employed in obtainingpolyvinyl alcohol as a dry powder. We have found that in many instancesdrying the polyvinyl alcohol causes it to case harden and thisintereferes with remelting when the photographic emulsion is beingprepared. It is also more difiicult to put dry powder into solution thanit is to melt a gel and, therefore, the preparation of photographicemulsions is facilitated by employing the highly purfied polyvinylalcohol in gel form.

We claim:

1. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate while in a alcohol solution bythe aid or an alkali deacetylating catalyst to form a polyvinyl alcoholgel, continuing the deacetylation to produce a gel which is insoluble inwater having a temperature not greater than approximately 50 C., andwashing the gel with water having a temperature not greater thanapproximately 50 C. until substantially free from the reactionbyproducts and residual catalyst.

2. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in methanol containing notmore than 15% water by the aid of an alkali deacetylating catalyst toform a polyvinyl alcohol gel which is insoluble in water having atemperature not greater than approximately 50 C. and washing the gelwith water having a temperature not greater than approximately 50 C.until substantially free from reaction by-products and residualcatalyst.

3. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate while in a methanol solutionby the aid of a deacetylating catalyst selected from the groupconsisting of sodium hydroxide, potassium hydroxide, sodium methoxide,and lithium hydroxide to form a polyvinyl alcohol gel, continuing thedeacetylation to produce a gel which is insoluble in water having atemperature not greater than approximately 50 C. and Washing the gelwith water having a temperature not greater than approximately 50 C.until substantially free from the reaction by-products and residualcatalyst.

4. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of a deacetylation catalyst selected from the group consistingof sodium hydroxide, potassium hydroxide, sodium methoxide, and lithiumhydroxide to form a polyvinyl alcohol gel, permitting the gel tosynerese, separating the syneresed liquid from the gel, continuing thedeacetylation with fresh catalyst in alcoholic solution to removeresidual acetyl, and washing the thus formed water-insoluble polyvinylalcohol gel with water having a temperature not greater thanapproximately 50 C. until substantially free from other material.

5. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of an aqueous de acetylation catalyst selected from the groupconsisting of aqueous sodium hydroxide, aqueous potassium hydroxide, andaqueous lithium hydroxide to form a polyvinyl alcohol gel, permittingthe gel to synerese, separating the syneresed liquid from the gel,continuing the deacetylation with fresh deacetylation agent in aqueoussolution to remove residual acetyl, and washing the thus 152 formedwater-insoluble polyvinyl alcohol gel with water having a temperaturenot greater than approximately 50 C. until substantially free from othermaterial.

6. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of an alkali deacetylation catalyst selected from the groupconsisting of sodium hydroxide, potassium hydroxide, sodium methoxide,and lithium hydroxide to form a polyvinyl alcohol gel, permitting thegel to synerese, separating the syneresed liquid from the gel,continuing the deacetylation with fresh alkali catalyst in alcoholicsolution to remove additional acetyl, permitting the gel to drain, andcontinuing the deacetylation with fresh acid deacetylation catalyst inalcoholic solution, the acid being one selected from the groupconsisting of hydrochloric acid and sulfuric acid to form a gel which isinsoluble in water having a temperature not greater than approximately50 C. and washing the gel with water having a temperature not greaterthan approximately 50 C. until substantially free from reactionby-products and residual catalyst.

7. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of an alkali deacetylation catalyst selected from the groupconsisting of sodium hydroxide potassium hydroxide, sodium methoxide,and lithium hydroxide to form a polyvinyl alcohol gel, permitting thegel to synerese, separating the syneresed liquid from the gel andcontinuing the deacetylation with aqueous acid deacetylation catalyst,the acid being one selected from the group consisting of hydrochloricacid and sulfuric acid to form a gel which is insoluble in water havinga temperature not greater than approximately 50 C. and washing the gelwith water having a temperature not greater than approximately 50 C.until substantially free from reaction by-products and residualcatalyst.

8. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of an alkali deacetylation catalyst selected from the groupconsisting of sodium hydroxide, potassium hydroxide, sodium methoxide,and lithium hydroxide to form a polyvinyl alcohol gel, permitting thegel to synerese, separating the syneresed liquid from the gel,continuing the deacetylation with fresh alkali catalyst in alcoholicsolution to remove additional acetyl, permitting the gel to drain, andcontinuing the deacetylation with fresh alkali in aqueous solution, andfinally washing the gel with water having a temperature not greater thanapproximately 50 C. until substantially free from reaction by-productsand residual catalyst.

9. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of an alkali deacetylation catalyst selected from the groupconsisting of sodium hydroxide, potassium hydroxide, sodium methoxide,and lithium hydroxide to form a polyvinyl alcohol gel, permitting thegel to synerese, separating the syneresed liquid from the gel,continuing the deacetylation with fresh alkali catalyst in alcoholicsolution to remove additional acetyl, permitting the gel to drain, andcontinuing the deacetylation with aqueous acid deacetylation catalyst,the acid being one selected from the group consisting of hydrochloricacid and sulfuric acid to form a gel which is insoluble in water havinga temperature not greater than approximately 50 C. and washing the gelwith water having a temperature not greater than approximately 50 C.until substantially free from reaction by-products and residualcatalyst.

10. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of an alkali deacetylation catalyst selected from the groupconsisting of sodium hydroxide, potassium hydroxide, sodium methoxide,and lithium hydroxide to form a polyvinyl alcohol gel, permitting thegel to synerese, separating the syneresed liquid from the gel,continuing the deacetylation with fresh alkali catalyst in alcoholicsolution to remove additional acetyl, washing the gel with Water toremove reaction by-products and residual catalyst, and continuing thedeacetylation with aqueous acid, the acid being one selected from thegroup consisting of hydrochloric acid and sulfuric acid to form a gelwhich is insoluble in water having a temperature not greater thanapproximately 50 C. and washing the gel with water having a temperaturenot greater than approximately 50 C. until substantially free fromreaction by-products and residual catalyst.

11. The method of preparing a highly pure polyvinyl alcohol whichcomprises deacetylating polyvinyl acetate in alcoholic solution in thepresence of an alkali deacetylation catalyst selected from the groupconsisting of sodium hydroxide, potassium hydroxide, sodium methoxide,and lithium hydroxide to form a polyvinyl alcohol gel, permitting thegel to synerese, separating the syneresed liquid from the gel,continuing the deacetylation with fresh alkali catalyst in alcoholicsolution to remove additional acetyl, washing the thus formed gel withwater to remove reaction lay-products and unreacted catalyst, continuingthe deacetylation with fresh aqueous alkali, again washing the gel withwater, and continuing the deacetylation with aqueous acid, and finallywashing the gel with water, the water employed for the washings having atemperature not greater than approximately 50 C.

12. The method of preparing a highly pure polyvinyl alcohol whichcomprises subjecting polyvinyl acetate to a treatment with steam andthen deacetylating the polyvinyl acetate while in alcoholic solution bythe aid of an alkali deacetylating catalyst to form a gel which isinsoluble in Water and washing the gel with water having a temperaturenot greater than approximately 50 C. until substantially free from thereaction by-products and residual catalyst.

13. The method of preparing a highly pure and colorless polyvinylalcohol Which comprises treating an alcoholic solution of polyvinylacetate with hydrogen peroxide, deacetylating the thus treated polyvinylacetate with a deacetylating catalyst to form a polyvinyl alcohol gelwhich is insoluble in water having a temperature not greater thanapproximately 50 C. and washing the gel with water having a temperaturenot greater than approximately 50 C. until substantially free from thereaction by-products and residual catalysts.

14. The method of preparing a highly pure and colorless polyvinylalcohol which comprises deacetylating polyvinyl acetate while inmethanol solution by the aid of a deacetylating catalyst to form apolyvinyl alcohol gel, soaking the gel in a solution of sulfur dioxideand washing the gel with water having a temperature not greater thanapproximately 50 C. to remove sulfur dioxide, by-products of thereactions and residual catalyst.

References Cited in the file of this patent UNITED STATES PATENTS1,586,803 Hermann et al June 1, 1926 2,399,653 Roland May 7, 19462,467,774 Plambeck Apr. 19, 1949 2,642,420 Kenyon et al June 16, 1953FOREIGN PATENTS 747,879 Germany Oct. 18, 1944 OTHER REFERENCES Blaikieet al.: Ind. and Eng. Chem, vol. 18, October 1936, pages 1155-1158.

De Bell et al.: German Plastics Practice, Springfield, Mass, pages 111to 115, 1946.

Blout et al.: Journ. Amer. Chem. Soc., vol. 70, February 1948, pages862864.

1. THE METHOD OF PREPARING A HIGHLY PURE POLYVINYL ALCOHOL WHICHCOMPRISES DEACETYLATING POLYVINYL ACETATE WHILE IN A ALCOHOL SOLUTION BYTHE AID OF AN ALKALI DEACETYLATING CATALYST TO FORM A POLYVINYL ALCOHOLGEL, CONTINUING THE DEACETYLATION TO PRODUCE A GEL WHICH IS INSOLUBLE INWATER HAVING A TEMPERATURE NOTE GREATER THAN APPROXIMATELY 50* C., ANDWASHING THE GEL WITH WATER HAVING A TEMPERATURE NOT GREATER THANAPPROXIMATELY 50* C. UNTIL SUBSTANTIALLY FREE FROM THE REACTIONBYPRODUCTS AND RESIDUAL CATALYST.